The above information was helpful for me but the folder path information was incorrect. It basically required what you see in the Folders table column FolderPath. I was trying to delete some files and this is what I had to do:

String pdfFile = string.Format("{0}user.pdf",userId.ToString());
String imgFile = string.Format("{0}user.jpg", userId.ToString());
String folderPath = "Users/UserFiles/";
IFolderInfo folder = FolderManager.Instance.GetFolder(PortalId, folderPath);
if (folder != null)
{
IFileInfo file = FileManager.Instance.GetFile(folder, pdfFile);
if (file != null)
{
FileManager.Instance.DeleteFile(file); //pdf file
}
file = FileManager.Instance.GetFile(folder, imgFile);
if (file != null)
{
FileManager.Instance.DeleteFile(file); //img file
}
}